/*						rgamma.c
 *
 *	Reciprocal Gamma function
 *
 *
 *
 * SYNOPSIS:
 *
 * double x, y, rgamma();
 *
 * y = rgamma( x );
 *
 *
 *
 * DESCRIPTION:
 *
 * Returns one divided by the Gamma function of the argument.
 *
 * The function is approximated by a Chebyshev expansion in
 * the interval [0,1].  Range reduction is by recurrence
 * for arguments between -34.034 and +34.84425627277176174.
 * 1/MAXNUM is returned for positive arguments outside this
 * range.  For arguments less than -34.034 the cosecant
 * reflection formula is applied; lograrithms are employed
 * to avoid unnecessary overflow.
 *
 * The reciprocal Gamma function has no singularities,
 * but overflow and underflow may occur for large arguments.
 * These conditions return either MAXNUM or 1/MAXNUM with
 * appropriate sign.
 *
 * ACCURACY:
 *
 *                      Relative error:
 * arithmetic   domain     # trials      peak         rms
 *    DEC      -30,+30       4000       1.2e-16     1.8e-17
 *    IEEE     -30,+30      30000       1.1e-15     2.0e-16
 * For arguments less than -34.034 the peak error is on the
 * order of 5e-15 (DEC), excepting overflow or underflow.
 */

/*
Cephes Math Library Release 2.0:  April, 1987
Copyright 1985, 1987 by Stephen L. Moshier
Direct inquiries to 30 Frost Street, Cambridge, MA 02140
*/

#include "mconf.h"

/* Chebyshev coefficients for reciprocal Gamma function
 * in interval 0 to 1.  Function is 1/(x Gamma(x)) - 1
 */

#ifdef UNK
static double R[] = {
 3.13173458231230000000E-17,
-6.70718606477908000000E-16,
 2.20039078172259550000E-15,
 2.47691630348254132600E-13,
-6.60074100411295197440E-12,
 5.13850186324226978840E-11,
 1.08965386454418662084E-9,
-3.33964630686836942556E-8,
 2.68975996440595483619E-7,
 2.96001177518801696639E-6,
-8.04814124978471142852E-5,
 4.16609138709688864714E-4,
 5.06579864028608725080E-3,
-6.41925436109158228810E-2,
-4.98558728684003594785E-3,
 1.27546015610523951063E-1
};
#endif

#ifdef DEC
static unsigned short R[] = {
0022420,0066376,0176751,0071636,
0123501,0051114,0042104,0131153,
0024036,0107013,0126504,0033361,
0025613,0070040,0035174,0162316,
0126750,0037060,0077775,0122202,
0027541,0177143,0037675,0105150,
0030625,0141311,0075005,0115436,
0132017,0067714,0125033,0014721,
0032620,0063707,0105256,0152643,
0033506,0122235,0072757,0170053,
0134650,0144041,0015617,0016143,
0035332,0066125,0000776,0006215,
0036245,0177377,0137173,0131432,
0137203,0073541,0055645,0141150,
0136243,0057043,0026226,0017362,
0037402,0115554,0033441,0012310
};
#endif

#ifdef IBMPC
static unsigned short R[] = {
0x2e74,0xdfbd,0x0d9f,0x3c82,
0x964d,0x8888,0x2a49,0xbcc8,
0x86de,0x75a8,0xd1c1,0x3ce3,
0x9c9a,0x074f,0x6e04,0x3d51,
0xb490,0x0fff,0x07c6,0xbd9d,
0xb14d,0x67f7,0x3fcc,0x3dcc,
0xb364,0x2f40,0xb859,0x3e12,
0x633a,0x9543,0xedf9,0xbe61,
0xdab4,0xf155,0x0cf8,0x3e92,
0xfe05,0xaebd,0xd493,0x3ec8,
0xe38c,0x2371,0x1904,0xbf15,
0xc192,0xa03f,0x4d8a,0x3f3b,
0x7663,0xf7cf,0xbfdf,0x3f74,
0xb84d,0x2b74,0x6eec,0xbfb0,
0xc3de,0x6592,0x6bc4,0xbf74,
0x2299,0x86e4,0x536d,0x3fc0
};
#endif

#ifdef MIEEE
static unsigned short R[] = {
0x3c82,0x0d9f,0xdfbd,0x2e74,
0xbcc8,0x2a49,0x8888,0x964d,
0x3ce3,0xd1c1,0x75a8,0x86de,
0x3d51,0x6e04,0x074f,0x9c9a,
0xbd9d,0x07c6,0x0fff,0xb490,
0x3dcc,0x3fcc,0x67f7,0xb14d,
0x3e12,0xb859,0x2f40,0xb364,
0xbe61,0xedf9,0x9543,0x633a,
0x3e92,0x0cf8,0xf155,0xdab4,
0x3ec8,0xd493,0xaebd,0xfe05,
0xbf15,0x1904,0x2371,0xe38c,
0x3f3b,0x4d8a,0xa03f,0xc192,
0x3f74,0xbfdf,0xf7cf,0x7663,
0xbfb0,0x6eec,0x2b74,0xb84d,
0xbf74,0x6bc4,0x6592,0xc3de,
0x3fc0,0x536d,0x86e4,0x2299
};
#endif

static char name[] = "rgamma";

extern double PI, MAXLOG, MAXNUM;


double rgamma(x)
double x;
{
double w, y, z;
int sign;

if( x > 34.84425627277176174)
	{
	mtherr( name, UNDERFLOW );
	return(1.0/MAXNUM);
	}
if( x < -34.034 )
	{
	w = -x;
	z = sin( PI*w );
	if( z == 0.0 )
		return(0.0);
	if( z < 0.0 )
		{
		sign = 1;
		z = -z;
		}
	else
		sign = -1;

	y = log( w * z ) - log(PI) + lgam(w);
	if( y < -MAXLOG )
		{
		mtherr( name, UNDERFLOW );
		return( sign * 1.0 / MAXNUM );
		}
	if( y > MAXLOG )
		{
		mtherr( name, OVERFLOW );
		return( sign * MAXNUM );
		}
	return( sign * exp(y));
	}
z = 1.0;
w = x;

while( w > 1.0 )	/* Downward recurrence */
	{
	w -= 1.0;
	z *= w;
	}
while( w < 0.0 )	/* Upward recurrence */
	{
	z /= w;
	w += 1.0;
	}
if( w == 0.0 )		/* Nonpositive integer */
	return(0.0);
if( w == 1.0 )		/* Other integer */
	return( 1.0/z );

y = w * ( 1.0 + chbevl( 4.0*w-2.0, R, 16 ) ) / z;
return(y);
}
